1. Field of the Invention
The present invention relates to a differential signaling structure adopting a differential signaling system, in which a radiation noise from an electronic instrument is reduced.
2. Related Background Art
In recent years, in a signal transmission between electronic instruments, it is necessary to improve a data transmission rate so as to be compatible with a high-speed operation of the electric instruments. To improve the data transmission rate, there are required a higher frequency of a signal to be transmitted and a higher switching speed of the devices used for the signal transmission. To comply with the higher frequency and the higher speed of the signal to be transmitted, it is necessary to take measures against radiation noises. For this reason, a differential signaling system has been used in place of a conventional signaling system of a single-ended signaling system. In particular, a low voltage differential signaling (LVDS) system has a great effect in reducing the radiation noises because a signal wave form thereof is a low-amplitude voltage, in addition to the effect of canceling magnetic fields generated by anti-phase currents flowing through a differential signal pair with other.
FIG. 8 is a schematic diagram showing a general circuit structure adopting the LVDS system. In FIG. 8, reference numeral 1000 denotes a printed circuit board, reference numeral 100 denotes a transmission side circuit element, reference numeral 101 denotes a reception side circuit element, and reference numeral 300 denotes a ground pattern. Reference numeral 1001 denotes a printed wiring board, and the transmission side circuit element 100 and the reception side circuit element 101 are mounted on the printed wiring board 1001. Between the transmission side circuit element 100 and the reception side circuit element 101, a differential signaling line 8 is arranged by providing signaling lines 1 and 2 that have the same electrical characteristics, thereby performing differential signaling with a low-amplitude voltage.
A terminating resistor 3 having a value substantially equal to a differential impedance of the differential signaling line is provided between in the vicinity of input terminals of the reception side circuit element 101 and connected to the signaling lines 1 and 2. By providing the terminating resistor 3, the entire anti-phase currents flowing through the signaling lines 1 and 2 are thermally consumed, thereby making it possible to suppress the distortion of a wave form and the generation of radiation noises due to reflection. The signaling lines 1 and 2 are arranged to be adjacent in substantially parallel to each other and have the same length. As a result, the anti-phase currents flowing through the signaling lines 1 and 2 generate magnetic fields having substantially the same quantity in opposite directions to be cancelled out, thereby making it possible to suppress generation of the radiation noises.
In FIG. 8, in addition to the differential signaling line 8 for transmitting a high frequency signal, there are provided three low frequency signaling lines 4, 5 and 6, and a ground line 7. The low frequency signaling lines 4, 5, and 6 are connected to the transmission side circuit elements 200, 202 and 204, respectively, and to the reception side circuit elements 201, 203 and 205, respectively, and transmit a signal having an extremely small frequency as compared with the differential signaling line 8. The signal transmitted through the low frequency signaling lines 4, 5 and 6 have a small frequency, so that the radiation noise is not a problem even by adopting the transmission system of the single-ended signaling system. Both ends of the ground pattern line 7 each are connected to a ground pattern 300, and the ground line 7 constitutes return paths for the differential signaling line 8 and the low frequency signaling lines 4, 5 and 6.
The differential signaling system represented by the LVDS system is effective in reducing the radiation noises due to the high frequency signal. However, to comply with the higher frequency and the higher speed of the signal, a standard for the radiation noises becomes more stringent year after year, so that the differential signaling system is not sufficient enough to deal with the radiation noises.
Even when two signaling lines of the differential signaling line are designed to have completely the same electrical characteristics, an in-phase current component is generated in the differential signaling line due to a time lag within the transmission side circuit element, a difference between build up and build down characteristics thereof, and the like. The differential signaling system is effective for an anti-phase signal, but not capable of suppressing the radiation noises generated due to an in-phase signal. The radiation noise generated in the differential signaling line due to the in-phase current component is called a common mode noise.
In a case of a circuit structure shown in FIG. 8, the in-phase current component flows through the differential signaling line 8, thereby generating the common mode noise. With regard to the in-phase current component flowing from the transmission side circuit element 100 to the reception side circuit element 101, there is no path through which the in-phase current component flows past the reception side circuit element 101. As a result, the in-phase current component returns to the transmission side circuit element 100 through a stray capacitance and the like of the printed wiring board while straying, whereby the radiation noises is generated.
Japanese Patent Application Laid-Open No. H11-205118 proposes an application of a center tap terminal circuit to a differential signaling system shown in FIG. 9. In FIG. 9, reference numerals 10 and 11 denote resistors which are designed to have about a half value of a differential impedance of a differential signaling line. The resistors 10 and 11 are connected in series to signaling lines 1 and 2 between the signaling lines 1 and 2 in the vicinity of input terminals of the reception side circuit element 101. Reference numeral 12 is a capacitor provided between a connection point of the resistors 10 and 11 connected in series and a ground pattern 300 to connect the connection point to the ground pattern. An in-phase current component generated in the differential signaling lines 1 and 2 flows to the ground pattern 300 through the resistors 10 and 11 having the same value, and the capacitor 12. Then, the in-phase current component returns to a reception side circuit element 100 through a ground line 7, which is connected to the ground pattern, as a return path. Thus, it is possible to suppress the radiation noise.
Japanese Patent Application Laid-Open No. 2001-007458 proposes an application of a center tap terminal circuit to a differential signaling system shown in FIG. 10. In FIG. 10, the radiation noise is reduced by devising an arrangement of in-phase current components and ground lines which become return paths for the current components. In FIG. 10, reference numerals 13 and 14 denote ground lines that are newly provided to be adjacent to and in substantially parallel to signaling lines 1 and 2, and that are each connected to the ground pattern 300. With this structure, the in-phase current components generated in the signaling lines 1 and 2 flow through a center tap terminal constituted of the resistors 10 and 11 and the capacitor 12, and then returns to the reception side circuit element 100 through the two ground lines 13 and 14. In this case, the magnetic fields generated by the in-phase current component flowing through the signaling lines 1 and 2 and the magnetic fields generated by the return current flowing through the ground lines 13 and 14 cancel out each other in the vicinity of a pole, thereby making it possible to reduce the radiation noise.
However, in the case of the differential signaling system shown in FIG. 9, when the ground line 7 serving as the return path for the in-phase current is apart from the signaling lines 1 and 2, a current loop becomes large. As a result, the effect of reducing the radiation noise is not obtained sufficiently.
In addition, in the case of the differential signaling system shown in FIG. 10, it is necessary to add two ground lines each time of adding a pair of differential signaling lines. This increases the number of connector pins and cable cores. Further, this causes an increase in packing density and contour size of a printed wiring board on which the connector is mounted, an increase in cross-sectional area of the cable, and the like, thereby increasing a manufacturing cost of a circuit, and preventing an electric instrument from being small-sized. A higher speed system requires more lines which need to be switched to the differential signaling system, thereby making the problems more serious.